Fastenerless internal support for hollow structures

ABSTRACT

A reinforced structural member having a tubular member and a support structure. The tubular member has a sidewall that defines a hollow interior portion. The support structure is formed from a composite material and bonded to the sidewall in at least two predetermined locations. The support structure is positioned within the tubular member and configured to transmit force between the predetermined locations so as to prevent the tubular member from being crushed in response to the application of a force of a predetermined magnitude to the tubular member. A method for forming a reinforced structural member is also provided.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

[0001] This invention was made with Government support under ContractNumber DAAJ09-91-C-A004 awarded by DOD/Army. The government has certainrights in this invention.

TECHNICAL FIELD

[0002] The present invention relates generally to structural members andmore particularly to a strong yet lightweight hollow structural memberand a method for its formation.

BACKGROUND OF THE INVENTION BACKGROUND ART

[0003] In the design and fabrication of the various types of structuralmembers that are employed in the aerospace industry, it is frequentlydesirable to minimize the weight of the device by incorporating a hollowchamber into the structural member. One drawback associated with thisapproach is that the tubular or extruded components that are typicallyutilized in the fabrication of light weight structural members aredifficult to mechanically fasten to other structures or devices as theclamping force that is generated by the mechanical fastener is often ofsufficient magnitude to crush the tubular or extruded component.

[0004] One solution that has been proposed for resisting the tendencyfor the mechanical fastener to crush the tubular or extruded componentis the use of metallic support structures or potting compounds tofacilitate the transmission of load across the tubular or extrudedcomponent through a path that has sufficient strength andcross-sectional area to prevent the clamping force from crushing thetubular or extruded component. The drawbacks associated with bothconventionally-installed metallic support structures and pottingcompounds are similar in that their placement within the tubular orextruded component requires that the tubular or extruded component befabricated in several pieces to permit the support structure or pottingcompound to be introduced and accurately positioned within the hollowinterior. A typical design incorporating this concept is illustrated inFIG. 1.

[0005] As illustrated, the structural member 2 includes a first shellmember 3, a second shell member 4, a plurality of support structures 5and a plurality of mechanical fasteners 6, which are typically rivets.In fabricating the structural member 2, the support structures 5 arepositioned within an internal cavity 3 a formed in the first shellmember 3 and fastened thereto with several of the mechanical fasteners6. The second shell member 4 is then positioned over the first shellmember 3, thereby encapsulating the support structures 5 within thehollow cavity defined by the first and second shell members 3 and 4. Theremaining mechanical fasteners 6 are then employed to fixedly couple thesecond shell member 4 to the first shell member 3. The degree of overlapbetween the first and second shell members 3 and 4 that is necessary topermit these two components to be coupled together, as well as the useof mechanical fasteners 6 to both position the support structures 5 andfasten the first and second shell members 3 and 4 together, addsconsiderable weight to the structural member 2.

[0006] Accordingly, there remains a need in the art for a structuralmember and a method for its construction wherein one or more supportstructures are accurately positioned and secured within the hollowinterior of a tubular or extruded component without the use ofmechanical fasteners.

SUMMARY OF THE INVENTION

[0007] In one preferred form, the present invention provides areinforced structural member having a tubular member and a supportstructure. The tubular member has a sidewall that defines a hollowinterior portion. The support structure is formed from a compositematerial and bonded to the sidewall in at least two predeterminedlocations. The support structure is positioned within the tubular memberand configured to transmit force between the predetermined locations soas to prevent the tubular member from being crushed in response to theapplication of a force of a predetermined magnitude to the tubularmember.

[0008] In another preferred form, the present invention provides amethod for fabricating a structural member. The method includes thesteps of: providing a tubular member having a sidewall that defines ahollow interior portion; providing a support insert formed from acomposite material, the support insert being configured to fit withinthe hollow interior and contact the sidewall in at least twopredetermined locations; positioning the support insert within thehollow interior at a predetermined location; and bonding the supportinsert to the tubular member such that the support insert is operablefor transmitting a load between the predetermined locations.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Additional advantages and features of the present invention willbecome apparent from the subsequent description and the appended claims,taken in conjunction with the accompanying drawings, wherein:

[0010]FIG. 1 is a structural member constructed in accordance with theteachings of the prior art;

[0011]FIG. 2 is a partial sectional view of a structural memberconstructed in accordance with the teachings of the present invention;

[0012]FIG. 3 is an cross-sectional view of a portion of the structuralmember taken along the line 3-3 of FIG. 2 and illustrating theconstruction of the support structures;

[0013]FIG. 4 is an exploded perspective view of a portion of thestructural member illustration the construction of the supportstructure;

[0014]FIG. 5 is a schematic illustration of the tooling employed in thefabrication of the structural member of FIG. 2; and

[0015]FIG. 6 is a sectional view of a structural member constructed inaccordance with an alternate embodiment of the present invention takenalong the longitudinal axis of the structural member.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0016] With reference to FIG. 2 of the drawings, a reinforced structuralmember constructed in accordance with the teachings of the presentinvention is generally indicated by reference numeral 10. In theparticular example provided, the structural member 10 is a helicoptertransmission support strut that is secured to a transmission housing 12via a plurality of conventional mechanical fasteners, such as a threadedbolt 14 and nut 16. Those skilled in the art will understand that theparticular embodiment illustrated is merely exemplary and that theteachings of the present invention have applicability to other aerospaceand non-aerospace applications.

[0017] The structural member 10 is illustrated to include a tubularmember 20 and at least one support structure 22. The tubular member 20has a sidewall 30 that defines a hollow interior portion 32. The tubularmember 20 is preferably unitarily formed so as to minimize the weight ofthe structural member 10 and may be a conventionally formed tubular orextruded component. A plurality of through-holes 34 are formed throughthe sidewall 30 to receive the bolts 14.

[0018] With additional reference to FIGS. 3 and 4, the support structure22 is formed from a support insert 40 which is bonded to and curedwithin the hollow interior portion 32 of the tubular member 20 (i.e.,the support structure 22 is a support insert 40 which has beenimpregnated with an epoxy resin and cured). Those skilled in the artwill understand that the particular configuration of the supportstructure 22 will vary depending upon various design criteria, includingthe magnitude and types of loads that are to be transmitted through thetubular member 20 and the cross-sectional shape of the hollow interiorportion 32. Accordingly, those skilled in the art will understand thatthe particular configuration illustrated is merely exemplary and notintended in any way to limit the scope of the present invention.

[0019] With specific reference to FIGS. 3 and 4, the construction of thesupport structure 22 and support insert 40 is illustrated in greaterdetail. In the particular example provided, the support insert 40includes a pair of rib members 44 and a tube member 46. The rib members44 and the tube member 46 are formed from woven or unidirectional carbonfiber, but may be formed from any suitable composite material includingfiberglass, carbon graphite or Kevlar®. In the particular exampleprovided, the tube member 46 is formed from 10 plies of an AS4 or T300woven carbon fiber, with the plies having an orientation of [ ]₁₀ andthe rib members 44 are formed from 6 piles of an AS4 or T300 wovencarbon fiber, with their plies having an orientation of [45/0/45]_(s).The thickness of the plies that form the tube member 46 is about 0.08inch, while the thickness of the plies that form each of the rib members44 is about 0.05 inch.

[0020] Each of the rib members 44 is initially formed as a laid-up sheetmember 50 which is thereafter tackified and placed in a forming die 52.The forming die 52 is employed to simultaneously shape and heat thesheet member 50 to thereby produce a rib member 44 having sufficientstrength to facilitate the fabrication of the support insert 40 and thestructural member 10. In the particular embodiment illustrated, each ribmember 44 is illustrated to include a body portion 60 and a pair of endportions 62. The body portion 60 is shown to include an arcuate centerportion 64 that is configured to mate with a portion of the tube member46. Each end 66 of the body portion 60 is fixedly coupled to the arcuatecenter portion 64 at a first end and to one of the end portions 62 at asecond end. The intersections between the ends 66 and both the arcuatecenter portion 64 and the end portions 62 are illustrated to include apocket radii 68 which is employed to inhibit the formation of stressrisers in the support structure 22. Each end portion 62 has a generallyvertical central portion 70 and a pair of tips 72. Each tip 72 is shownto extend away from the central portion 70 and inwardly toward the bodyportion 60 in a manner which permits the end portions 62 to conform tothe sidewall 30 of the tubular member 20.

[0021] Unlike the rib members 44, the tube member 46 is impregnated withan epoxy and fully cured so as to prevent it from crushing during thesubsequent stages of the formation of the structural member 10. The tubemember 46 includes an aperture 80 that is sized to receive the bolt 14.The tube member 46 may simply be bonded to the arcuate center portions64 of the rib members 44 to form the support insert 40. It is preferred,however, that several pieces of a uni-directional roving material 84 bebonded to one of the rib members 44 to eliminate the presence of voidsthat would otherwise occur as a result of the use of pocket radii 68 inthe rib members 44 and the back-to-back placement of the rib members 44.

[0022] The support insert 40 is inserted into the hollow interiorportion 32 of the tubular member 20 and a suitable resin transfermolding process is employed to inject resin into the rib members 44. Theresin is then cured to form a support structure 22 which is securelybonded to the tubular member 20 in a manner that permits the clampingforce that is generated by the bolt 14 to be transmitted betweenpredetermined locations in the tubular member 20 without substantiallydeforming the hollow interior portion 32. Those skilled in the art willreadily understand that construction of support structure 22 in thismanner net match molds the outer mold line and the inner mold linesurfaces in a single curing operation.

[0023] With reference to FIGS. 2 and 5, the tooling for the fabricationof the structural member 10 is schematically illustrated. The tooling isshown to include a pair of end mandrels 100, a resin injection system102 and a vacuum source 104 and a mold 105. A spacer member 106 that isformed from a soluble material, such as salt, is employed to preciselyspace the support inserts 40 apart. The spacer member 106 and thesupport inserts 40 are positioned within the hollow interior portion 32of the tubular member 20 such that the aperture 80 of each of the tubemembers 46 is aligned using a removable tooling pin (not shown) to acorresponding one of the through-holes 34 that are formed through thesidewall 30 of the tubular member 20.

[0024] The end mandrels 100 are then inserted to the hollow interiorportion 32 of the tubular member 20 and fixedly coupled to the tubularmember 20. The end mandrels 100 contact an associated one of the supportinserts 40 and collectively cause the spacer member 106 to abut both ofthe support inserts 40. The assembly (i.e., tubular member 20 with theend mandrels 100) are placed between the mold halves 105 a that form themold 105 such that the mold halves 105 a engage the end mandrels 100.

[0025] The vacuum source 104 is next coupled to the tubular member 20and any air that is trapped within the hollow interior portion 32between the end mandrels 100 is removed. The resin injection system 102is thereafter employed to inject a molten epoxy resin into the supportinserts 40. The particular epoxy resin employed in this example is aPR500 one-part epoxy manufactured by the Minnesota Mining andManufacturing Company. Once injected, the epoxy resin is then curedunder a set of predetermined conditions, permitting the outer mold line(OML) and inner mold line (IML) of the support structure 22 to be formedby net match molding and cured in a single operation. The predeterminedconditions may include the heating of the epoxy to a predeterminedtemperature and/or exposing the epoxy to a predetermined level ofhydrostatic pressure. In the example provided, the predeterminedconditions included the heating of the epoxy to a temperature of about350° F. and the application of about 80 p.s.i. to about 100 p.s.i. ofhydrostatic pressure to the epoxy for a period of about 2 hours. Itshould be noted that the use of the end mandrels 100, the spacer member106 and the tubular member 20 permits all of the outer surfaces of thesupport structure 22 to be formed to a predetermined thickness to ensurethe structural integrity of the support structure 22.

[0026] After the resin has cured, the tubular member 20 is removed fromthe mold 105. Thereafter, the end mandrels 100, resin transfer system102 and vacuum source 104 are removed from the tubular member 20 and thestructural member 10 is immersed in a suitable liquid to dissolve thespacer member 106. Alternatively, as shown in FIG. 6, the spacer member106 may be fabricated from a lightweight structural material, such asfoam, to eliminate the need to remove the spacer member 106 from thepocket between the support structures 22. In the embodiment illustrated,the foam is Rohacell Foam that has been machined to fit within thehollow interior portion 32 and abut the support inserts 40.

[0027] While the invention has been described in the specification andillustrated in the drawings with reference to a preferred embodiment, itwill be understood by those skilled in the art that various changes maybe made and equivalents may be substituted for elements thereof withoutdeparting from the scope of the invention as defined in the claims. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment illustrated by the drawingsand described in the specification as the best mode presentlycontemplated for carrying out this invention, but that the inventionwill include any embodiments falling within the foregoing descriptionand the appended claims.

What is claimed is:
 1. A method for fabricating a structural member, themethod comprising the steps of: providing a tubular member having asidewall that defines a hollow interior portion; providing a supportinsert formed from a composite material, the support insert beingconfigured to fit within the hollow interior and contact the sidewall inat least two predetermined locations; positioning the support insertwithin the hollow interior at a predetermined location; and bonding thesupport insert to the tubular member such that the support insert isoperable for transmitting a load between predetermined locations.
 2. Themethod of claim 1, wherein the step of bonding the support insert to thetubular member includes the steps of: introducing an epoxy resin to thesupport insert; and curing the epoxy resin under a set of predeterminedconditions.
 3. The method of claim 2, wherein the set of predeterminedconditions includes heating the epoxy resin to a predeterminedtemperature.
 4. The method of claim 3, wherein the predeterminedtemperature is about 350° F.
 5. The method of claim 2, wherein the setof predetermined conditions further includes applying a predeterminedlevel of hydrostatic pressure to the epoxy resin.
 6. The method of claim5, wherein the predetermined level of hydrostatic pressure is about 80p.s.i. to about 100 p.s.i.
 7. The method of claim 1, wherein before thestep of bonding the support insert to the tubular member the methodfurther includes the steps of: providing a second support insert formedfrom a composite material, the second support insert being configured tofit within the hollow interior and contact the sidewall in at least twopredetermined locations; providing a spacer member; and positioning thespacer member and the second support insert within the hollow interiorat a predetermined location, the spacer member being disposed betweenand abutting the support insert and the second support insert.
 8. Themethod of claim 7, wherein after the step of bonding the support insertto the tubular member the method includes the step of removing thespacer member.
 9. The method of claim 8, wherein the spacer member isformed from salt.
 10. The method of claim 7, wherein the spacer memberis formed from foam.
 11. The method of claim 1, wherein the step ofproviding the support insert includes the steps of: providing a sheetmember formed from the composite material; tackifing the sheet member;providing a forming die; and introducing the sheet member in the die toform the sheet member and generate at least a portion of the supportinsert.
 12. The method of claim 11, wherein the step of providing thesupport insert further includes the step of bonding a precured compositemember to the formed sheet member.
 13. The method of claim 12, whereinthe precured composite member is tubular in shape.
 14. The method ofclaim 12, wherein the step of providing the support insert furtherincludes the step of bonding a unidirectional fiber within a voidlocated between the precured composite member and the formed sheetmember.
 15. A reinforced structural member comprising: a tubular memberhaving a sidewall that defines a hollow interior portion; and a supportstructure formed from a composite material and bonded to the sidewall inat least two predetermined locations, the support structure beingpositioned within the tubular member and configured to transmit forcebetween the predetermined locations so as to prevent the tubular memberfrom being crushed in response to the application of a force of apredetermined magnitude to the tubular member.
 16. The reinforcedstructural member of claim 15, wherein the support structure includes atubular component having a generally hollow interior.
 17. The reinforcedstructural member of claim 16, wherein the support structure furtherincludes a pair of generally C-shaped rib structures, each of the ribstructures conforming to at least a portion of the tubular component,each of the rib structures being bonded to the other one of the ribstructures and the tubular component.
 18. The reinforced structuralmember of claim 17, wherein the support structure includes a pluralityof uni-directional fibers, each of the unidirectional fibers beingbonded to at least one of the rib structures to fill a void formedbetween a respective one of the rib structures and at least one of theother one of the rib structures and the tubular component.
 19. Thereinforced structural member of claim 15, wherein the support structureis formed from a composite material selected from a group of compositematerials consisting of fiberglass, carbon fibers and Kevlar.
 20. Thereinforced structural member of claim 15, wherein the support structureis bonded to the tubular member in a resin transfer molding operation.21. A method for fabricating a structural member, the method comprisingthe steps of: providing a tubular member having a sidewall that definesa hollow interior portion; and molding a support at a predeterminedlocation within the hollow interior portion of the tubular member, thesupport being net match molded to an outer mold line and an inner moldline and cured in a single operation.